This invention relates to a method for manufacturing magnetostrictive materials. More particularly, the invention is concerned with a method of inexpensively manufacturing rod-shaped rare earth-iron magnetostrictive materials having good purity and quality.
Since the 1970's rare earth-iron magnetostrictive materials having a Laves structure of RE-Fe.sub.2 have attracted considerable interest since they can create large room temperature magnetostriction due to their high Curie points. They promise to be suitable for a wide variety of technological applications including generation of ultrasonic waves in sonar, as drive elements in small transducers and actuators, as vibration dampers, and as switches or valves. Many attempts have been proposed to manufacture such materials.
U.S. Pat. No. 4,308,474 discloses a grain-oriented polycrystalline or single crystal rare earth-iron magnetostrictive material of the formula Tb.sub.x Dy.sub.1-x Fe.sub.2-w wherein 0.20.ltoreq.x .ltoreq.1.00 and 0.ltoreq.w.ltoreq.0.20. The rare earth metals may be partly replaced by Sm and/or Ho.
U.S. Pat. No. 4,378,258 discloses another rare earth-iron magnetostrictive material of the formula R.sub.x T.sub.1-x wherein R is a rare earth or mixtures thereof, T is Fe, Ni, Co, Mn, or mixtures thereof, and 0&lt;x&lt;1.
Other publications disclosing rare earth-iron magnetostrictive materials include Japanese Patent Publication No. 61-33892(1986) and Japanese Patent Application Kokai No. 64-73050(1989).
U.S. Pat. No. 4,152,178 discloses a sintered rare earth-iron magnetostrictive product with a grain oriented morphology manufactured by the powder metallurgy method. The grain orientation is created by magnetically aligning powder particles of the magnetostrictive material prior to sintering. The product is essentially polycrystalline.
U.S. Pat. No. 4,609,402 describes the manufacture of a single-crystal rod of a rare earth-iron magnetostrictive material by a free-standing zone melting (FSZM) method. This method provides a product with excellent magnetostrictive properties. However, the productivity of the method is rather low.
U.S. Pat. No. 4,770,704 describes a modified Bridgman method in which a grain-oriented polycrystalline magnetostrictive material rod can be manufactured more efficiently. According to this method, a previously-prepared alloy which is usually in the form of buttons or fingers is melted by induction heating in a crucible having a bottom outlet. The melt is discharged through the outlet and deposited in an elongated mold which is either fixed or movable downwardly. Heat is removed from the deposited melt through the lower end portion of the mold to progressively solidify the melt from the bottom. The solid-liquid interface of the melt moves upwardly toward the top of the mold. The unidirectional solidification of the melt produces axial grain orientation in the product.
Other methods which can be applied to the manufacture of a magnetostrictive material having a Laves structure include powder bonding and casting.
Since an optimum magnetostriction of a magnetostrictive material of the Laves structure is obtained along the &lt;111&gt; direction, it is desirable that the grains be aligned or oriented in the &lt;111&gt; direction. In the above-mentioned FSZM method and modified Bridgman method, however, it is difficult to prepare magnetostrictive materials having &lt;111&gt; alignment along the longitudinal axis of the rod. Instead, these methods tend to grow crystals with &lt;112&gt; alignment. Therefore, &lt;112&gt; aligned materials have been used for practical purposes.
According to the powder metallurgy method, it is possible to obtain &lt;111&gt; aligned magnetostrictive materials by magnetic aligning. However, it is difficult to align all the powder particles in this manner. As a result, the magnetostrictive properties of a product prepared by powder metallurgy are inferior to those of a &lt;112&gt; aligned polycrystalline or single crystal rod prepared by the FSZM or modified Bridgman method.
In the FSZM, modified Bridgman, and powder metallurgy methods, a previously-prepared alloy having the same composition as the product has to be used as a starting material in order to assure that the product has a uniform microstructure. A powdered alloy is used in the powder metallurgy method, while an alloy in the form of buttons or fingers is usually used in the other methods.
Thus, prior to the manufacture of the magnetostrictive materials, the metallic raw materials must be melted together in a crucible to form an alloy having a uniform composition. For this purpose, either high-frequency induction melting or arc melting can be employed.
Induction melting greatly activates the rare earth metals in the raw materials. Therefore, when a crucible for commercial use is used, the rare earth metals tend to react with the material which constitutes the crucible and it is practically impossible to avoid contamination of the product with the crucible material, resulting in degradation in the magnetostrictive properties of the product. For this reason, alloying is frequently performed by arc melting of the raw materials in a water-cooled copper hearth to form an alloy in the shape of a small button or finger. This method, however, is not suitable for commercial-scale production of the alloy.
Regardless of the alloying method, the formation of an alloy from metallic raw materials in a separate step, i.e., the use of a preliminary alloying step adds to the manufacturing costs of the magnetostrictive materials.
Casting is an inexpensive method which can be applied to the manufacture of a magnetostrictive material. For this purpose, metallic raw materials are melted by arc heating or induction heating and then solidified in a mold. However, this method tends to produce a polycrystalline product with randomly oriented grains unless a special technique is employed to cause unidirectional solidification. As a result, the properties of the product are much inferior to a &lt;112&gt; aligned polycrystalline or single crystal rod obtained by the FSZM or modified Bridgman method. For example, the magnetostriction of the product is as low as one-third to one-tenth that of the aligned rod.
A unidirectional solidification technique has been applied to the manufacture of industrial materials such as turbine blades of a superalloy, e.g., a Ni-based superalloy. The powder down method and the high-rate solidification method have been developed to perform unidirectional solidification. However, these methods usually involve melting of the material by induction heating. Therefore, it is difficult to adopt these methods in the manufacture of a magnetostrictive material, since contamination of the product with the crucible material may occur as described above.